The synthesis of 3-methyl-3-pentene-2-one (3M3P) involves an aldol condensation reaction between acetaldehyde and methyl ethyl ketone using an acid catalyst. There is limited information available in the literature for this specific reaction. Heiba and Anderson (1959) used carbon tetrachloride for this reaction and reported acetaldehyde conversion of 32% with significant formation of acetaldehyde polymers. U.S. Pat. No. 4,234,518 describes the use of zinc acetate as catalyst for this reaction to give a mixture of 4-hexene-3-one and 3M3P in 31:69 ratio with 38% overall yield.
The industrial process for manufacture of 3M3P generally uses mineral acid catalysts such as sulfuric acid to drive the reaction and is typically conducted in a semi-batch or batch mode in a stirred tank reactor. In such reactor system, the reaction medium needs to be stirred vigorously to overcome the mass transfer limitations between organic reactants and aqueous acid phases to try and maximize the product yield and reduce reaction time. The intensity of mixing is usually a limiting factor in production scale batch reactors, thus resulting in low reactor throughput and product yields. In addition, the process using mineral acids generate huge aqueous waste streams that need to be disposed. Furthermore, due to relatively high water solubility, significant amount of product and reactants are being lost in the aqueous stream, thus further lowering product yield and adding to environmental concerns.
One approach that addresses some of these issues is the use a solid acid catalyst rather than a mineral acid for this chemical transformation. Mahajan et al (2007) studied the use of Amberlyst 15, a solid acid catalyst and developed a kinetic model and Mahajani et al (2009) used this model in a computer simulation exercise to propose a reaction-distillation system for 3M3P. However, the high residence times (60 hours at 70-75° C.) based on simulation results is impractical to be of any value for industrial applications. Typically, higher temperatures are used to increase reaction speed and lower residence time however, in this case higher temperatures (>100° C.) are detrimental to the Amberlyst 15 catalyst, reduces catalyst life and increases cost making the approach uneconomical. Snell et al (2010) have reported the use of solid aluminophosphate catalysts, but the yield of 3M3P they obtained (<6%) was too low to be of practical value.
For practical industrial application of the solid acid catalyst, the combination of a good catalyst with long life, process and operating conditions to achieve high yield of 3M3P in a reasonable reaction time and appropriate reactor design all play a critical role. The present invention discloses a practical 3M3P process using a continuous reactor system and solid acid catalyst with good activity and long life that gives high product yield with low residence time, and essentially no waste.